Compare Value in the Database table

Hi,

i am new to C# coding, i have a userprofile database table(SQL server 2005). In this database table i have columns "username"AND  "date".

i want to iterate through the all the records in the this table and to check whether the current logged on user on the site is present in the database table or not , if present i need to display message "user is present." or call other xxx method.
DataSet dt = new DataSet();foreach(DataTable table in dt.Tables)
{
fo ...